Abstract

We have applied a combination of x-ray diffraction and x-ray-absorption spectroscopy to probe, in a self-consistent way, the bulk and local structural changes, respectively, associated with the mechanical alloying of pure metallic powders of Ni and Mo. From a careful analysis using an analytical atomic-subshell, we show that the Ni (fcc) atoms are totally involved in the alloying reaction with Mo (bcc) atoms taking up substitutional positions in the Ni (fcc) lattice. Prior to the formation of the amorphous phase, at milling times ≈10 h, an intermediate and highly distorted NiMo phase forms with an internal strain of ≈2.1×10-3. The release of this strain with milling time corresponds to an increase in the Mo content in the alloy and the nucleation of an amorphous phase together with a subsequent loss of long-range order of the Ni lattice. The resultant amorphous phase closely resembles δ-NiMo and after milling times ≈54 h has a nominal composition of Ni50Mo38. Its formation is associated with Mo in a residual but distorted bcc environment characterized by a particle size of ≈70 nm and an internal strain of 1.8×10-3. The detailed local structure around Ni atoms during this process is presented and discussed.